Taxonomy: Ensure consistency of `hide_empty` in term queries when `ta…

…xonomy` is excluded.
When querying for terms in hierarchical categories using `hide_empty=true`,
results have historically included parent terms which are themselves
unattached to any objects (are "empty") but which have non-empty descendent
terms. Because this process involves walking the descendant tree, we avoid it
when we detect that the queried taxonomies are not hierarchical. (This
behavior was introduced in [5525].)
When the `taxonomy` parameter of `get_terms()` was made optional - see #35495,
[36614] - it affected the mechanism for avoiding unneccessary tree walks,
since there may not be any explicitly declared taxonomies to run through
`is_taxonomy_hierarchical()`. As a result, term queries excluding `taxonomy`
did not check descendants, and empty parents with non-empty children were not
included in `hide_empty` results.
We correct the behavior by crawling term descendants when the `taxonomy`
argument is absent, which means that we're querying for terms in all taxonomies.
Props smerriman.
Fixes #37728.
git-svn-id: https://develop.svn.wordpress.org/trunk@45888 602fd350-edb4-49c9-b593-d223f7449a82